scala - Spark 源代码 : How to understand withScope method

Question

我无法理解 withScope 方法的功能(实际上，我并不真正了解 RDDOperationScope 类的含义)

特别是withScope方法的参数列表中(body: => T)是什么意思:

private[spark] def withScope[T](
  sc: SparkContext,
  name: String,
  allowNesting: Boolean,
  ignoreParent: Boolean)(body: => T): T = {
// Save the old scope to restore it later
val scopeKey = SparkContext.RDD_SCOPE_KEY
val noOverrideKey = SparkContext.RDD_SCOPE_NO_OVERRIDE_KEY
val oldScopeJson = sc.getLocalProperty(scopeKey)
val oldScope = Option(oldScopeJson).map(RDDOperationScope.fromJson)
val oldNoOverride = sc.getLocalProperty(noOverrideKey)
try {
  if (ignoreParent) {
    // Ignore all parent settings and scopes and start afresh with our own root scope
    sc.setLocalProperty(scopeKey, new RDDOperationScope(name).toJson)
  } else if (sc.getLocalProperty(noOverrideKey) == null) {
    // Otherwise, set the scope only if the higher level caller allows us to do so
    sc.setLocalProperty(scopeKey, new RDDOperationScope(name, oldScope).toJson)
  }
  // Optionally disallow the child body to override our scope
  if (!allowNesting) {
    sc.setLocalProperty(noOverrideKey, "true")
  }
  body
} finally {
  // Remember to restore any state that was modified before exiting
  sc.setLocalProperty(scopeKey, oldScopeJson)
  sc.setLocalProperty(noOverrideKey, oldNoOverride)
}
}

您可以通过以下链接找到源代码:
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rdd/RDDOperationScope.scala

谁能帮我？谢谢，我纠结了很久。

Answer 1

以下代码可能对您有所帮助

object TestWithScope {
    def withScope(func: => String) = {
        println("withscope")
        func
    }

    def bar(foo: String) = withScope {
        println("Bar: " + foo)
        "BBBB"
    }

    def main(args: Array[String]): Unit = {
        println(bar("AAAA"));
    }
}

可能的输出

withscope
Bar: AAAA
BBBB